1. Field of the Invention
The present invention relates to coloring by electrolysis of aluminum or aluminum alloys and, more particularly, to the use of a treating bath containing a certain aliphatic sulfonic acid and a metal salt of such sulfonic acid in the coloring by electrolysis.
2. Description of the Prior Art
Heretofore, known methods of coloring aluminum or aluminum alloys include a method of coloring a chemically formed coating on the aluminum and a method of coloring an anodized coating on the aluminum or aluminum alloy. Particularly, with respect to the latter there may be mentioned primary electrolytic coloring methods, secondary electrolytic coloring methods, and methods which comprise forming an anodized coating on aluminum and then coloring the coating. Among the primary methods are a spontaneous coloring method and a method of using an organic acid. As secondary methods, a method of electrolyzing with an alternating current in an acidic solution of a heavy metal salt is well known (Japanese Patent Application Publication No. 1715/1963).
However, these prior art methods have a number of disadvantages. For example, in the method of coloring the chemicaly formed coatings, the life of the treating liquid is short, the resistance to corrosion and the strength of the coating are poor, and the stability of the treating liquid is low due to the use of a strong oxidizing agent such as chromates, bichromates, permanganates and the like, thereby producing uneveness of coloring and changes in color tones. Further it is impossible to control the treating liquid and color tones of the coating. The hardness of the colored coating obtained is also sufficient. On the other hand, the above-mentioned method of coloring by electrolysis with an alternating current has eliminated these disadvantages but still has difficulty coloring uniformly and exhibits poor covering power. Later, this method was improved by adding a tin salt to an electrolyzing bath (Japanese Patent Application Publication No. 37823/1971). However, when the amount of an article to be treated is increased, the stability of the liquid and the reproducibility of color tones are reduced, and ultimately white precipitates form and the liquid turns yellow so that it is unable to function.
The reason why this bath is unstable is that stannous sulfate per se is liable to undergo hydrolysis to produce insoluble precipitates. This tendency is notable particularly in a dilute acidic tin bath. In order to prevent this tendency, organic acids such as cresol sulfonic acid, phenol sulfonic acid and the like have been added to the bath in a small amount. However, oxidation of tin still could not be sufficiently controlled. Further, the use of these phenol sulfonic acid type agents cause serious environmental problems which is a fatal defect.
We made studies to obviate the disadvantages and defects of the prior art methods mentioned above, and have now found that, by using a certain aliphatic sulfonic acid such as unsubstituted or hydroxy-substituted alkane sulfonic acids together with a metal salt of such sulfonic acids as a principal component in an electrolyzing bath used for the production of colored Alumite, the stability of the electrolyzing bath is remarkably improved and particularly superior uniform coloring can be obtained. It has also been found that the colored coating obtained has superior resistances to corrosion, light, sunlight and abrasion, high flexural strength and superior workability.
Further, as compared with phenol sulfonic acid type agents these aliphatic sulfonic acids or metal salts thereof have little problem in the prevention of environmental pollution and at the same time give a number of better results. When subjected to electrolysis for a long period of time, a tin salt of a hydroxy-containing sulfonic acid produces no white precipitates due to the oxidation mentioned above. The reason for this is believed to be that the hydroxy-containing alkane sulfonic acid prevents the metal salt from oxidizing because the sulfonic acid per se is a reducing acid.
Accordingly, it is an object of the present invention to provide a method of forming a colored coating having excellent resistances to corrosion, sunlight, heat and the like on aluminum or aluminum alloys by electrolyzing aluminum or alloys in a bath containing a certain aliphatic sulfonic acid and a metal salt of such sulfonic acid, which bath has high stability, provides a clear and uniform coloring and produces no environmental pollution.